A solid-state imaging device is a television camera comprising a photosensitive target surface which is formed by a CCD (charge coupled device) array. According to the principles of this device, a minor number of carriers among electron-hole pairs which correspond to the projected light intensity are stored inside a well of CCD electrode potential, and these stored charges are transferred to a non-photosensitive part of the CCD and further forwarded to an output device to be converted to video signals. The transfer is controlled to be in synchronization with a horizontal and a vertical retrace line period.
FIG. 11 is a cross-sectional view showing an element of a conventional solid-state imaging device. In FIG. 11, reference numeral 1 represents an n-type substrate; 2 represents a light-intercepting part; 3 represents a p-type buried diffusion layer; 4 represents a p-type impurity layer; 5 represents a transfer part; 6 represents a p-type readout control part; 7 represents a channel stopper; 8 represents an insulating film; 9 represents a conductive-type electrode; and 10 represents a p-type well.
The n-type substrate 1 has the p-type well 10 inside, and the p-type well 10 includes the n-type light-intercepting part 2 which produces and stores signal charges, the p-type impurity layer 4 which is positioned next to this light-intercepting part 2, and the n-type transfer part 5 which transfers signal charges stored inside this p-type impurity layer 4, wherein all these parts are formed in array. This structure is hereinafter referred to as a vertical OFD structure. The readout side between the light-intercepting part 2 and the transfer part 5 is separated by the p-type readout control part 6 which is installed to control threshold voltage (Vt), and the non-readout side is separated by the channel stopper 7. The insulating film 8 is formed on the light-intercepting part 2 and on the transfer part 5, and charges of the light-intercepting part 2 are read out to the transfer part 5 by the conductive-type electrode 9. Also, for the purpose of preventing dark current from arising, the p-type buried diffusion layer 3 is formed at the interface of the light-intercepting part 2 and the insulating film 8.
When formed in the vertical OFD structure, it is possible to discharge (excess) charges which are stored in the light-intercepting part 2 to the backside of the substrate by changing the substrate voltage.
FIG. 12 is a cross-sectional view showing an element of a conventional solid-state imaging device using a p-type substrate. In FIG. 12, reference numeral 2 represents a light-intercepting part; 3 represents a p-type buried diffusion layer; 4 represents a p-type impurity layer; 5 represents a transfer part; 6 represents a p-type readout control part; 7 represents a channel stopper; 8 represents an insulating film; 9 represents a conductive-type electrode; 11 represents a p-type substrate; 12 represents a horizontal excess charge control part; and 13 represents an n-type drain part.
The p-type substrate 11 includes the n-type light-intercepting part 2 which produces and stores signal charges, the p-type impurity layer 4 which is positioned next to this light-intercepting part 2, the n-type transfer part 5 which transfers signal charges stored inside this p-type impurity layer 4, and the n-type drain part 13 for discharging (excess) charges, and which is positioned next to the transfer part 5. This structure is hereinafter referred to as a horizontal OFD structure. The light-intercepting part 2 and the transfer part 5 are separated by the p-type readout control part 6 which is installed to control threshold voltage (Vt), and the transfer part 5 and the n-type drain part 13 are separated by the channel stopper 7, and furthermore, the light-intercepting part 2 and the n-type drain part 13 are separated by the horizontal excess charge control part 12 which is installed to control threshold voltage (Vt) for reading out (excess) charges. The insulating film 8 is formed on the light-intercepting part 2 and the transfer part 5, and also on the light-intercepting part 2 and the n-type drain part 13. Charges of the light-intercepting part 2 are read out to the transfer part 5 by the conductive-type electrode 9, and (excess) charges of the light-intercepting part 2 are read out to the n-type drain part 13. Furthermore, for the purpose of preventing dark current from arising, the p-type buried diffusion layer 3 is formed at the interface of the light-intercepting part 2 and the insulating film 8.
When formed in the horizontal OFD structure, it is possible to discharge (excess) charges which are stored in the light-intercepting part 2 to the n-type drain part 13 by changing the voltage of the conductive-type electrode 9 between the light-intercepting part 2 and the n-type drain part 13. See U.S. Pat. No. 5,233,429.
However, the vertical OFD structure in the conventional system has the problem of causing deterioration of sensitivity characteristics, because the signal charges produced by the absorption of long wavelength light in the substrate depth part of the light-intercepting part 2 are discharged to the side of the n-type substrate 1.
Furthermore, the problem with the horizontal OFD structure in the conventional system is that unit cell areas of the light-intercepting part 2 and the transfer part 5 etc. tend to become large in general since the n-type drain part 13 is needed for discharging (excess) charges. This is disadvantageous to miniaturization and is also accompanied by deterioration in the sensitivity and saturation characteristics along with the miniaturization.